The electromagnetic flow meter is a device with a well developed body of art in which techniques are available for measuring flow velocity of various types of fluids in various environments. In one environment in which the flow is confined to a conduit, the flow meter includes what is known as a spool type transducer in which an electromagnetic field is generated by a coil surrounding the conduit, and electrodes in contact with the flow have voltages induced therein representative of the flow velocity. Another type of known electromagnetic flow meter is of the probe type in which the flowing fluid is not confined to a conduit. In the probe type flow meter the transducer may take the form of a cylinder. The cylinder includes therein an electromagnetic coil for producing an electromagnetic field which is uniformly disposed about the periphery of the cylinder. Located in the vicinity of the surface of the cylinder are electrodes which have voltages induced therein representative of the flow velocity.
A constant source of problem in this field of technology is the low level of signals induced in the electrodes. For example, the spool type electrode is capable of generating signals on the order of 300 microvolts per foot per second of fluid flow velocity. The probe type transducer generates signals on the order of 25 microvolts per foot per second of fluid flow velocity. Obviously, voltages of this order of magnitude cannot be used directly and therefore it is conventional in the art to employ signal processing to increase the magnitude of the signal so that it may be utilized. A typical form of signal processing is shown in my U.S. Pat. Nos. 3,759,097 and 3,885,858.
Generally stated, the function of the signal processing circuitry is to focus on the flow induced voltages, and separate them from the effects of extraneous noise and amplify the flow induced voltages while ignoring the extraneous noise. While the signal processing techniques known in the art are quite effective, nevertheless, the extraneous voltages or noise voltages are still reflected in the output of the signal processing chain. The signal processing circuitry is effective to focus on the voltages periodically induced on the electrodes such that random noise which is of the same order of magnitude or even slightly greater than the flow induced voltages, is acceptedly rejected. However, when the noise signals are much greater than the flow induced voltages, they tend to saturate the active elements in the signal processing chain. The effect of saturation is to lose the output signal until the active elements come out of saturation. As a result, noise voltages of relatively large orders of magnitude cause signal dropouts which are disturbing to the user.
The triboelectric noise effects associated with flow measurements in dielectric fluids has been reduced, in the prior art, by operating the transducer at relatively high frequencies inasmuch as this noise spectrum decreases with frequency. However, low frequency operation of the transducer is desirable from a number of other standpoints. Therefore, it is important to be able to reduce the triboelectric noise or reduce its effects, with apparatus capable of relatively low frequency operation.